A typical passenger transit or subway type train includes a locomotive, a plurality of railcars and several trainlines. The trainlines include both pneumatic and electrical lines most of which run from the locomotive to the last railcar in the train. The main reservoir equalization (MRE) pipe is one such pneumatic trainline. It consists of a series of individual pipe lengths. Secured to the underside of each railcar, one such pipe length connects via a coupler to another such pipe length secured to a neighboring railcar. The MRE pipe is thus essentially one long continuous pipe that runs from the locomotive to the last railcar. Charged by various air compressors located throughout the train, it is the MRE pipe that serves to supply air to the various reservoirs, such as the supply reservoir, located on each railcar in the train.
One pneumatic trainline of particular importance to passenger transit and subway type trains is the brake pipe. It is used to convey to each railcar in the train an emergency brake signal when an emergency condition arises. Of similar importance is the brake control trainline that is used to carry the brake command to each railcar in the train as discussed below. Contained within a protective conduit along with other electrical trainlines, the brake control trainline is similarly formed from individual conduits connected in series.
A locomotive for a passenger transit or a subway type train typically has an electropneumatic brake control system such as the RT-5 Brake Control System produced by the Westinghouse Air Brake Company (WABCO). Adapted or configured to fit the needs of various passenger transit authorities, each of the RT-5 style systems currently in service feature a master controller by which a train operator can direct the overall braking and propulsive efforts for the entire train.
The master controller in the locomotive houses a handle, a computer and various other related components. The handle can be moved longitudinally anywhere along its range of motion and into any one of several designated positions. By moving the handle into the appropriate position, a train operator can initiate, maintain or halt braking or propulsion of the train. For example, from a position in which the train is currently being propelled, moving the handle to what is referred to as the full service position causes a service application of the brakes. Similarly, when moved to the emergency position, the operator can initiate an even faster type of braking referred to as an emergency application of the brakes. There are other positions for the handle whose purposes are beyond the scope of the invention described and claimed below.
Based on the positions of the handle, the computer of the master controller can ascertain whether, and to what degree, the overall braking or propulsive effort of the train should be reduced or increased. A keyboard may also be used to permit the operator greater access to the brake equipment, allowing, for example, input of set-up parameters. Other known components may also be used to provide various other signals to the computer.
Based on the inputs it receives and the software that dictates its operation, the master controller essentially controls the overall operation of the brakes. For service braking, the master controller formulates the brake command appropriate to current conditions and conveys it along the brake control trainline to each of the railcars in the train. Through its brake command, the master controller can order any action from a release of brakes to a service application of the brakes or any degree of brake application in between those two extremes. For emergency braking, a push-button type emergency valve in the locomotive can be used to effect a drop in brake pipe pressure to an emergency level using both pneumatic and electrical means simultaneously. When push-actuated, the emergency valve provides a path for the brake pipe to vent directly to atmosphere. It also simultaneously deenergizes an emergency trainline thereby deenergizing one or more emergency magnet valves to further vent the brake pipe. Alternatively, when directed by the master controller, an emergency brake control valve on the locomotive could be used to decrease brake pipe pressure to the emergency level. By reducing the brake pipe pressure to the emergency level, whether initiated from the locomotive or from any other point in the train, this sends an emergency brake signal along the brake pipe to all other railcars in the train.
On passenger transit and subway type trains, the brake pipe is typically operated according to a binary logic scheme. Normal operating pressure for the brake pipe during non-emergency situations ranges from 130 to 150 psi, the pressure to which it is charged via the MRE pipe. The transition point, or emergency level, lies at approximately 90 psi. A pressure of 90 psi or below indicates an emergency. It is this lower pressure range that constitutes the emergency brake signal.
Each passenger transit railcar typically includes an electronic controller and two trucks, with each truck typically having two axles. In response to the brake command received from the master controller in the locomotive, the electronic controller controls the operation of both trucks on the railcar. The electronic controller, however, has two central processing units (CPUs). Along with its associated interface equipment, each CPU controls the brake equipment of one truck independently of the other truck. It does so based on the brake command and various other inputs specific to the truck that it controls.
The brake equipment for a truck includes a pneumatic control unit and one or more pneumatically operated brake cylinders. Shown in FIG. 1, the pneumatic control unit typically houses an application magnet valve (AMV), a release magnet valve (RMV), a relay valve, an emergency transfer valve (ETV), a variable load valve (VLV) and an air spring pressure transducer. Used to convert the pressure received from a load sensing system on the truck, the air spring transducer provides a feedback signal indicative of the load borne by the truck.
The relay valve typically takes the form of a J-1 relay valve or similar type valve. It is an air piloted device whose construction and operation are well known in the brake control art. It features a control port connected to the ETV, a supply port supplied by the supply reservoir, an output port from which air can be directed from the supply reservoir to the brake cylinder(s) and an exhaust port from which to vent the brake cylinder(s) to atmosphere. The pressure of the air impinging upon its control port and the pressure of the air that the relay valve delivers to the brake cylinders will be approximately equal, though the air delivered by the latter will be in much greater quantity than that received by the former.
During non-emergency operation of the pneumatic control unit (i.e., when brake pipe pressure lies above the transition point), the ETV assumes an access state in which it connects the control port to both the AMV and RMV. The AMV when opened then allows air from the supply reservoir via the VLV to reach the control port. The RMV when opened allows whatever pressure that impinges on the control port to be vented to atmosphere. By selectively controlling the opening and closing of the AMV and RMV when the ETV is switched to the access state, the electronic controller can control the magnitude of the pressure received by the control port. A brake cylinder control transducer, also a part of the pneumatic control unit, converts the pressure at the control port to yet another feedback signal. Along with other signals such as those relating to speed, dynamic braking, wheel slip, the air spring feedback and others, this feedback signal is conveyed to the electronic controller to aid it in controlling each pneumatic control unit independently.
The electronic controller acts upon the brake command that it receives from the master controller in the locomotive. Specifically, during service braking, each CPU formulates the exact amount of braking effort appropriate for its truck. It does this by processing the brake command and the aforementioned other signals according to a brake control process whose specifics are beyond the scope of the invention described and claimed below. Operating in what can be referred to as a service braking mode when its ETV is switched to the access state, the pneumatic control unit has its AMV and RMV magnet valves controlled by their corresponding CPU; each magnet valve being energizable by the CPU with a field effect transistor (FET). By such control of the AMV and RMV magnet valves, the CPU can control the flow of air from the supply reservoir via the VLV and the AMV and RMV magnet valves to the control port via the ETV. This produces at the control port of the relay valve a low capacity pressure corresponding to the amount of braking effort formulated for that particular truck.
The pneumatic control unit operates in what can be referred to as an emergency braking mode when its ETV is switched to the bypass state. Specifically, in an emergency, the ETV responds to the emergency brake signal by pneumatically switching itself to the bypass state in which the AMV and RMV are cutoff from the control port. Air from the supply reservoir is then allowed to flow via the VLV through the ETV directly to the control port. Built at the control port of the relay valve in this manner is a low capacity pressure capable of initiating an emergency application of the brakes on the truck.
In response to whatever low capacity pressure is impinging on its control port, the relay valve provides to the brake cylinder(s) a corresponding pressure of high capacity. This compels the brake cylinder(s) to apply the brakes on the truck. The magnitude of the braking force applied to the wheels is directly proportional to the pressure built up in the brake cylinder(s). The brake cylinder(s) convert the pressurized air that they receive to mechanical force. This force is transmitted by mechanical linkage to the brake shoes. Forced against the truck wheels and/or disc brakes, the brake shoes are used to slow or stop the rotation of the wheels according to the braking effort sought via the brake command or the emergency brake signal.
It is also well known that the braking effort sought to be applied to wheels of a truck is often formulated to take into account the weight of the load borne by the truck through a process generally known as load compensation. This is the purpose of the VLV shown in FIGS. 1 and 2.
The variable load valve (such as that shown and described in Operation & Maintenance Publication 4229-1 published by WABCO) is an air piloted device whose construction and operation are well known in the brake control art. Entirely mechanical in nature, the VLV typically features two spring-biased piston assemblies, at least one of which adjustable, that are designed to work with the air pressure received from the load sensing system (i.e., air springs) on the truck. The magnitude of the air spring pressure is indicative of the load that the truck is currently carrying. The VLV is designed to limit the maximum pressure at which air from the supply reservoir is directed to the control port of the relay valve. This maximum control pressure level is proportional to the pressure that the VLV receives from the air springs. For any particular level of air spring pressure, the VLV determines the maximum allowable pressure that will be supplied to the control port of the relay valve in an emergency.
Regarding the combined operation of the VLV and the relay valve, when the pneumatic control unit operates in the emergency braking mode, its ETV is in the bypass state thereby bypassing the AMV and RMV valves and allowing air to flow from the VLV directly to the control port. The control port thus receives the maximum allowable pressure (i.e., emergency brake control pressure) that the VLV can provide based on the load that the truck is currently carrying. As is apparent from FIG. 3, the VLV is essentially set so that the emergency brake control pressure for an empty railcar is X psi and for a fully loaded railcar it is (X+Y) psi. Though the emergency brake control pressure can vary from X to (X+Y) psi depending on the load borne by the railcar at any given time, it will never decrease below X or increase beyond (X+Y). The relay valve responds to the emergency brake control pressure by pressurizing the brake cylinder(s) to an emergency pressure level, a level determined by the setting of the VLV. When operating in the service braking mode with its ETV in the access state, the pneumatic control unit has its AMV and RMV valves controlled by their corresponding CPU. By manipulating the AMV and RMV valves according to aforementioned brake control process, the CPU produces at the control port a lower capacity pressure (i.e., a service brake control pressure) corresponding to the amount of braking effort formulated for that particular truck. The magnitude of the service brake control pressure is determined by the CPU according to the aforementioned brake control process. The relay valve responds to the service brake control pressure by pressurizing the brake cylinder(s) to a service pressure level, a level determined by the CPU and one that will never exceed the emergency brake control pressure setting of the VLV. In this manner, the VLV allows the truck to be braked at a relatively constant rate under fluctuating passenger loads.
The variable load valve has certain disadvantages when compared to the invention described and claimed below. First, the VLV is inherently compromised in its reliability due to its purely mechanical nature. It is a device that necessarily requires many parts, properly assembled and maintained, to perform its intended function, each part being subject to mechanical wear and tear. Second, the VLV occupies a comparatively large amount of space in, and adds weight to, the system into which it is incorporated. The pneumatic piping that is necessary to connect the VLV to and from the pneumatic components in the system in which it is employed also occupies space in, and adds weight to, the system. The design of the VLV has basically reached its zenith, it cannot be feasibly reduced much in size or in weight. Weight and space are two especially important factors in the rail industry where the costs of fuel and the capability to transport cargo or passengers affect the viability of railroad and passenger transit authorities alike.
The foregoing background information is provided to assist the reader to understand the invention described and claimed below. Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.